Cleaned up to match Drupal standards + clean up fields on disable

Hey,

First of all, thanks! I've been using your tutorial to set up gift certificates on a client's project, and it's been really helpful.

I did a little clean up to bring it closer in line with Drupal code standards. I added some code to remove the fields and line item when the module's disabled, and I also moved the code that creates the line items and its fields into a separate .install file. I figured I'd do the courtesy of offering my changes back.

Let me know if you've got any questions. Thanks!